Brønsted acid-catalyzed selective C–C bond cleavage of 1,3-diketones: a facile synthesis of 4(3H)-quinazolinones in aqueous ethyl lactate

Abstract

A facile and green approach was developed for the synthesis of 4(3H)-quinazolinones by using camphorsulfonic acid as a catalyst in an aqueous solution of biodegradable ethyl lactate. Various 2-aryl-, 2-alkyl-, and 2-(4-oxoalkyl)quinazolinones were obtained by cyclization of 2-aminobenzamides with a wide range of acyclic or cyclic 1,3-diketones via C–C bond cleavage in satisfactory to excellent yields.

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Introduction

The direct selective cleavage of unstrained C–C bonds has attracted much attention and emerged as a challenging transformation in organic synthesis due to the high C–C bond strength.¹ As inexpensive and readily available starting materials, 1,3-diketones have been widely used as important substrates in organic synthesis.² In 2010, Lei reported the first example of CuI-catalyzed C–C bond cleavage of 1,3-diketones and arylation to give α-arylketones.^3a Esters,^3b α-ketoesters,^3c amides,^3d,3e α-ketoamides^3f or α-amino acid esters^3g could be obtained from the reactions of 1,3-diketones with alcohols or amines via C–C bond cleavage in the presence of a Lewis acid, or under oxidative conditions. Recently, CuI-catalyzed tandem cyclization of o-halobenzoic acids,^3h esters,³ⁱ or amides^3j with 1,3-diketones leading to isocoumarins was developed. Very recently, H₂O₂-promoted reactions of aliphatic primary amines with 1,3-diketones for the synthesis of 1H-pyrrol-3(2H)-ones have also been realized.^3k

4(3H)-Quinazolinones are building blocks of many naturally occurring alkaloids and marketed drugs (Fig. 1).⁴ Owing to their importance and utility, a range of synthetic methods have been developed to construct quinazolinone derivatives.^5–7 It should be noted that most reported methods for the preparing of 4(3H)-quinazolinones request expensive transition-metal catalysts in the presence of oxidants and bases under harsh reaction conditions. Therefore, more environmentally benign and efficient methods to approach valuable quinazolinone derivatives are highly desirable.

Fig. 1 Selected examples of alkaloids and marketed drugs incorporating 4(3H)-quinazolinone cores.

Ethyl lactate is prepared by esterification of ethanol with lactic acid, both of which can be obtained by fermentation of biomass. Ethyl lactate has recently attracted much attention and has been used in organic synthesis as an environmentally benign and biodegradable solvent.⁸ Continuing our research interest in green catalysis,⁹ we have discovered a green approach for the synthesis of 4(3H)-quinazolinones by cyclization of 2-aminobenzamides with a wide range of acyclic or cyclic 1,3-diketones via C–C bond cleavage in the presence of camphorsulfonic acid (CSA) as a Brønsted catalyst in biodegradable ethyl lactate solution under metal-, oxidant-, and radiation-free conditions (Scheme 1).

Scheme 1 A green approach to 4(3H)-quinazolinones.

Results and discussion

For our initial studies, we chose the reaction of 2-aminobenzamide (1a) with pentane-2,4-dione (2A) as a model process for optimizing the reaction conditions (Table 1). No reaction was observed when amide 1a was treated with dione 2A in poly(ethylene glycol) (PEG-400) at 100 °C in the absence of a catalyst (Table 1, entry 1). However, the desired product 3aA was obtained in various yields on adding 10 mol% of a Brønsted acid catalyst to the reaction mixture (entries 2–6). Among the tested Brønsted acid catalysts, p-toluenesulfonic acid (TsOH·H₂O), acetic acid (AcOH), and trifluoroacetic acid (F₃CCO₂H) gave yields of less than 10%. Moderate yields of quinazolinone 3aA were obtained when trifluoromethanesulfonic acid (F₃CSO₃H; entry 5) or natural camphorsulfonic acid (CSA; entry 6) was used as catalyst, with CSA providing the higher yield (61%). When other green solvents such as PEG-200, ethyl lactate, and water were screened in this transformation, ethyl lactate gave the best results (entries 6–9). To our delight, we obtained product 3aA in up to 98% yield by using a mixture of ethyl lactate and water as the solvent (entry 11). Decreasing the catalyst loading from 10 mol% to 5 mol% resulted in a relatively low yield (entry 12). We therefore performed subsequent reactions of 2-aminobenzamides with various 1,3-diketones in the presence of 10 mol% camphorsulfonic acid as catalyst at 100 °C in a 1 : 9 (v/v) mixture of ethyl lactate and water for 16–24 hours.

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst	Solvent	Yield^b
a Reaction conditions: 2-aminobenzamide (1a; 0.2 mmol), pentane-2,4-dione (2A; 0.3 mmol), catalyst (10 mol%), solvent (1.0 mL), 100 °C, 16 h.b Isolated yield.c No reaction.d CSA: camphorsulfonic acid.e 5 mol% of CSA was use.
1	None	PEG-400	NR^c
2	TsOH·H2O	PEG-400	10%
3	AcOH	PEG-400	<5%
4	F3CCO2H	PEG-400	<5%
5	F3CSO3H	PEG-400	48%
6	CSA^d	PEG-400	61%
7	CSA	PEG-200	42%
8	CSA	Ethyl lactate	74%
9	CSA	H2O	43%
10	CSA	Ethyl lactate–H2O (1 : 1)	84%
11	CSA	Ethyl lactate–H2O (1 : 9)	98%
12	CSA^e	Ethyl lactate–H2O (1 : 9)	81%

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With the optimized reaction conditions in hand, we examined the reactions of 2-aminobenzamide (1a) with various acyclic 1,3-diketones. As shown in Table 2, a lower yield of 3aB comparing with that of 3aA was observed when the reaction of 1a and heptane-3,5-dione (2B) was carried out under the optimized conditions (3aA: 98%; 3aB: 81%). The reaction of amide 1a with sterically hindered 2,6-dimethylheptane-3,5-dione (2C) provided the desired product 3aC in moderate yield (3aC: 59%). In contrast, the reactants remained unchanged in the attempted reaction of amide 1a and the more sterically hindered 2,2,6,6-tetramethylheptane-3,5-dione (2D), even on raising the temperature and prolonging the reaction time. 2-Phenylquinazolin-4(3H)-one (3aE) was obtained by the reaction of 1a with 1,3-diphenylpropane-1,3-dione (2E) in 79% yield. Finally, the reactions of 1a with the unsymmetrical 1,3-diketones 1-phenylbutane-1,3-dione (2F), 5,5-dimethylhexane-2,4-dione (2G), and 1,1,1-trifluoropentane-2,4-dione (2H) were examined, all gave the same product, 3aA, in 54–75% yield through selective C–C bond cleavage. These results indicate that the reactivity of the acetyl group is higher than that of the benzoyl, pivaloyl, or trifluoroacetyl group.

Table 2 The scope of acyclic 1,3-diketones 2^a

Entry	2	3	Yield^b
a Reaction conditions: 2-aminobenzamide (1a; 0.2 mmol), pentane-2,4-dione (2; 0.3 mmol), CSA (10 mol%), H2O–ethyl lactate (9 : 1; 1.0 mL), 100 °C, 16 h.b Isolated yield.
1	R^1 = R^2 = Me, 2A	3aA	98%
2	R^1 = R^2 = Et, 2B	3aB	81%
3	R^1 = R^2 = i-Pr, 2C	3aC	59%
4	R^1 = R^2 = t-Bu, 2D	3aD	<1%
5	R^1 = R^2 = Ph, 2E	3aE	79%
6	R^1 = Me, R^2 = Ph, 2F	3aA	75%
7	R^1 = Me, R^2 = t-Bu, 2G	3aA	71%
8	R^1 = Me, R^2 = CF3, 2H	3aA	54%

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Next, we examined the scope of the reaction with respect to the 2-aminobenzamides. Various N-substituted 2-aminobenzamides 1a–m were treated with pentane-2,4-dione (2A) under the optimized conditions (Scheme 2). From the reaction of 2-amino-N-methylbenzamide (1b), the desired product 2,3-dimethylquinazolin-4(3H)-one (3bA) was isolated in 76% yield. The N-aryl-2-aminobenzamides with electron-donating groups (1d; 4-Me, 1e; 2-Me, and 1f; 4-MeO) or electron-withdrawing groups (1g; 3-Cl, 1h; 4-Cl, and 1i; 3,4-Cl₂) on the benzene ring also underwent the transformation to give the corresponding products 3cA–3iA in 56–93% yield. 3-Benzyl-2-methylquinazolin-4(3H)-one (3jA) was prepared in 88% yield by the reaction of 2-amino-N-benzylbenzamide with pentane-2,4-dione (2A). The corresponding reactions of 2-amino-6-fluorobenzamide and 2-amino-5-chlorobenzamide gave quinazolinones 3kA and 3mA in 88% and 79% yield, respectively.

Scheme 2 The scope of 2-aminobenzamides 1. Reaction conditions: 2-aminobenzamide (1; 0.2 mmol), pentane-2,4-dione (2A; 0.3 mmol), CSA (10 mol%), H₂O–ethyl lactate (9 : 1; 1.0 mL), 100 °C, 16 h, isolated yield.

Encouraged by these results, we extended the scope of diketone reactant to include cyclic 1,3-diketones 4A–G (Table 3). Treatment of 2-aminobenzamide (1a) with 1.5 equivalents of cyclohexane-1,3-dione (4A) in the presence of 10 mol% natural CSA in a 1 : 9 (v/v) mixture of ethyl lactate and water at 100 °C for 24 h gave 2-(4-oxopentyl)quinazolin-4(3H)-one (5aA) in 61% yield with full atom efficiency (Table 3, entry 1). When reactions of 1a with other substituted cyclohexane-1,3-diones 4B–E were carried out under the optimized conditions, the corresponding products 5aB–5aE were obtained in 35–84% yield. Interestingly, 2-methylcyclohexane-1,3-dione (4E) exhibited a higher reactivity than 5-methylcyclohexane-1,3-dione (4B) (entries 2 and 5). In contrast, cyclopentane-1,3-dione (4F) and 2-methyl-1,3-cyclopentane-1-3-dione (4G), which contain a five-membered ring, did not give the desired products 5aF and 5aG when treated with 1a (entries 6 and 7). We then examined the scope of the reaction with respect to the 2-aminobenzamides 1. In general, various 2-aminobenzamides 1 reacted with cyclohexane-1,3-dione (4A) under the optimized conditions to give the corresponding products 5bA–5kA in 55–98% yield (entries 8–17). With N-alkyl-substituted 2-aminobenzamides, products 5bA and 5jA were obtained in 78% and 87% yield, respectively (entries 8 and 16). N-Aryl-substituted 2-aminobenzamides also underwent this transformation. 2-Aminobenzamides with an electron-rich substituent on the aryl group gave products 5eA and 5fA in relatively high yields (entries 10 and 12), whereas electron-deficient aryl group-substituted 2-aminobenzamides gave products 5gA and 5iA in relatively low yields (entries 13 and 15). Finally, 2-amino-6-fluorobenzamide also reacted with cyclohexane-1,3-dione (4A) to give the corresponding product 5kA in 98%.

Table 3 Synthesis of 2-(4-oxoalkyl)quinazolinones 5^a

Entry	Substrate 1/4	Product 5	Yield^b
a Reaction conditions: 2-aminobenzamide 1 (0.2 mmol), cyclic 1,3-diketone 5 (0.3 mmol), CSA (10 mol%), 1 : 9 (v/v) ethyl lactate–H2O (1.0 mL), 100 °C, 24 h.b Isolated yield.c Not detected.
1	1a/4A		61%
2	1a/4B		35%
3	1a/4C		49%
4	1a/4D		38%
5	1a/4E		84%
6	1a/4F		ND^c
7	1a/4G		ND^c
8	1b/4A		78%
9	1c/4A		60%
10	1d/4A		96%
11	1e/4A		69%
12	1f/4A		84%
13	1g/4A		55%
14	1h/4A		80%
15	1i/4A		62%
16	1j/4A		87%
17	1k/4A		98%

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Based on the results obtained above and the literature,¹⁰ a proposed reaction mechanism was shown in Scheme 3. The Brønsted acid catalyzed condensation reaction of 2-aminobenzamide 1 with 1,3-diketone 4 would take place to generate a ketimine intermediate A, followed by tautomerization to give the enaminone intermediate B. Then, the intramolecular nucleophilic addition of B would produce adduct C. The C–C bond cleavage reaction would finally occur to generate the desired product 5.

Scheme 3 A proposed reaction mechanism.

Conclusions

In summary, we have developed an efficient, metal- and oxidant-free green approach for the synthesis of 4(3H)-quinazolinones. Various 2-aryl-, 2-alkyl-, and 2-(4-oxoalkyl)quinazolinones were prepared by successive condensation of 2-aminobenzamides with a wide range of acyclic or cyclic 1,3-diketones, intramolecular nucleophile addition, and selective C–C bond cleavage, catalyzed by natural camphorsulfonic acid in an aqueous ethyl lactate solution. Further applications of this methodology to synthesis of other N-heterocycles are currently underway in our laboratory.

Experimental section

General remarks

All reagents and solvents were purchased from commercial suppliers and used without further purification unless otherwise stated. Analytic thin-layer chromatography (TLC) was carried out with silica gel GF 254-coated plates. All products were isolated by column chromatography on silica gel (300–400 mesh) using petroleum ether (PE; bp 60–90 °C) and ethyl acetate. All compounds were characterized by ¹H NMR (400 MHz), ¹³C NMR (100 MHz), and ESI-MS. All ¹H NMR shifts are reported in δ units (ppm) relative to the signals for residual CHCl₃ (δ = 7.26 ppm) or DMSO (δ = 2.50 ppm) in the corresponding deuterated solvent. All ¹³C NMR spectra are reported in ppm relative to CDCl₃ (77.23 ppm) or DMSO-d₆ (39.60 ppm). NMR data were recorded on a Bruker 400 MHz instrument. HRMS data were recorded with ESI ionization sources on a Bruker Apex II instrument. Melting points were determined on an X-4 apparatus.

2-Methylquinazolin-4(3H)-one (3aA); typical procedure

A flask was charged with 2-aminobenzamide (1a; 27.2 mg, 0.2 mmol), pentane-2,3-dione (2A; 30.0 mg, 0.3 mmol), CSA (4.6 mg, 0.02 mmol), and 1 : 9 (v/v) ethyl lactate–H₂O (1.0 mL). The flask was sealed and the mixture was stirred at 100 °C for 16 h. When the reaction was complete (TLC), the mixture was cooled to r.t., extracted with EtOAc (3 × 20 mL), and washed with H₂O. The organic phase was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel) to give the product 3aA (31.7 mg, 98%) as white solid; mp: 286–288 °C; ¹H NMR (400 MHz, CDCl₃): δ = 2.61 (3H, s), 7.45–7.49 (1H, m), 7.68 (1H, d, J = 7.6 Hz), 7.76 (1H, dt, J₁ = 8.4 Hz, J₂ = 1.6 Hz), 8.28 (1H, dd, J₁ = 8.0 Hz, J₂ = 1.2 Hz), 12.19 (s, H); ¹³C NMR (100 MHz, CDCl₃): δ = 22.1, 120.2, 126.2, 126.4, 127.0, 134.9, 149.4, 153.3, 164.4; HRMS (ESI): m/z [M + H]⁺ calcd for C₉H₉N₂O: 161.0709; found: 161.0714.

2-(4-Oxopentyl)quinazolin-4(3H)-one (5aA); typical procedure

A flask was charged with 2-aminobenzamide (1a; 27.2 mg, 0.2 mmol), cyclohexane-1,3-dione (4A; 33.6 mg, 0.3 mmol), CSA (4.6 mg, 0.02 mmol), and 1 : 9 (v/v) ethyl lactate–H₂O (1.0 mL). The flask was sealed and the mixture was stirred at 100 °C for 24 h. When the reaction was complete (TLC), the mixture was cooled to r.t., extracted with EtOAc (3 × 20 mL), and washed with H₂O. The organic phase was dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel) to give the product 5aA (28.1 mg, 61%) as white solid; mp: 145–146 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 2.13–2.20 (m, 2H), 2.18 (s, 3H), 2.64 (t, J = 7.2 Hz, 2H), 2.80 (t, J = 7.2 Hz, 2H), 7.48 (dt, J₁ = 8.0 Hz, J₂ = 1.2 Hz, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.77 (dt, J₁ = 8.0 Hz, J₂ = 1.6 Hz, 1H), 8.29 (dd, J₁ = 8.0 Hz, J₂ = 1.2 Hz, 1H), 11.52 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz): δ = 21.1, 30.0, 34.7, 42.3, 120.6, 126.3, 126.5, 127.2, 134.8, 149.2, 155.8, 163.8, 208.2; HRMS (ESI): m/z [M + H]⁺ calcd for C₁₃H₁₄N₂O₂: 231.1128; found: 231.1134.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (21202092) and for a Startup Foundation from the China Three Gorges University (KJ2012B080, KJ2014H008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, compound characterizations, and copies of ¹H NMR and ¹³C NMR spectra. See DOI: 10.1039/c5ra17969f

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